The chip type variable resistor which represents the variable electronic parts includes, as described in patent document 1 and as conventionally well known, an insulating substrate formed in a chip type with a through hole provided at a central portion of the substrate, a resistance film provided on an upper surface thereof in an arcuate shape concentric with the through hole, an external terminal electrode corresponding to the respective end portions of the arcuate resistance film provided on the insulating substrate, and an internal terminal electrode plate made of a metal plate adhered to a lower surface of the insulating substrate and including an integrally formed shaft portion that fits in the through hole. On the upper surface side of the insulating substrate, an adjustment rotor made of a metal plate is rotatably mounted on an upper end portion of the shaft portion, and the rotor is constituted of a first plate formed in a bowl shape to receive a screwdriver that rotates the rotor, and a second plate superposed on an upper surface of the first plate and integrally coupled thereto via a fold-back joint. The first plate is provided with a sliding piece disposed in contact with the resistance film, and a screwdriver engagement hole in which the screwdriver is to be fitted is perforated in the second plate.
Also, in the conventional chip type variable resistor, as described in the patented document 1, an internal terminal electrode plate disposed on the lower surface of the insulating substrate is provided with a stopper piece formed to project upward from the upper surface of the insulating substrate, so that when the rotor rotates the fold-back joint of the rotor is butted to the stopper piece, and a rotation angle of the rotor is thereby delimited.
In the foregoing chip type variable resistor, the adjustment rotor is, as already stated, constituted of the first plate formed in a bowl shape to receive the screwdriver that rotates the rotor, and the second plate superposed on the upper surface of the first plate and integrally coupled thereto via the fold-back joint, in which the first plate is provided with the sliding piece disposed in contact with the resistance film, and the screwdriver engagement hole in which the screwdriver is to be fitted is perforated in the second plate. In other words, rotating the second plate with the screwdriver inserted in the screwdriver engagement hole perforated therein causes the first plate provided with the sliding piece disposed in contact with the resistance film to rotate, with the sliding piece provided thereto maintained in contact with the resistance film, and hence the fold-back joint between the first plate and the second plate is of a laid down U-shape, and the rotation is transmitted from the second plate to the first plate via such fold-back joint, which leads to a problem that when rotating the rotor with the screwdriver, the fold-back joint of the rotor is deformed such that the first plate and the second plate are shifted from each other.
For this reason, the patented document 1 proposes increasing a widthwise dimension of the fold-back joint for preventing the deformation, however increasing the width of the fold-back joint not only makes the folding work of the fold-back joint more difficult but also incurs an increase in weight, and besides when delimiting the rotation angle of the rotor by blocking the fold-back joint with the stopper piece, the rotation angle range is reduced to the same extent as the increase in width of the fold-back joint.
Also, for delimiting the rotation angle of the rotor the fold-back joint is butted to the stopper piece, and hence the stopper piece has to have a sufficient projecting height from the upper surface of the insulating substrate to reach the fold-back joint, which incurs a problem that a strength of the stopper piece is reduced against tilting in a rotation direction of the rotor thus being deformed, caused by the rotation thereof.
To increase the strength of the stopper piece against tilting, a widthwise dimension thereof may be increased in a rotation direction of the rotor, however increasing the width of the stopper piece not only incurs an increase in weight, but also results in reduction in rotation angle of the rotor, to the same extent as the increase in width of the stopper piece.
Patent document 1: JP-A-H11-354307